The present invention relates to chemical-mechanical planarization of semiconductor wafers, and more particularly, to a chemical-mechanical planarization machine that locally adjusts the contour of the wafer to enhance the uniformity of the planarized surface on the wafer.
Chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d) processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a planarizing liquid 44 on the polishing pad 40. The polishing pad 40 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 44 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the wafer, or the planarizing liquid 44 may be a planarizing solution without abrasive particles that contains only chemicals to etch and/or oxidize the surface of the wafer. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 also has an underpad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 40. In one type of CMP machine, a drive assembly 26 rotates the platen 20 as indicated by arrow A. In another type of CMP machine, the drive assembly reciprocates the platen back and forth as indicated by arrow B. Since the polishing pad 40 is attached to the underpad 25, the polishing pad 40 moves with the platen 20.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier to impart axial and/or rotational motion (indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing pad 40. While the face of the wafer 12 presses against the polishing pad 40, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 42. As the face of the wafer 12 moves across the planarizing surface 42, the polishing pad 40 and the planarizing liquid 44 continually remove material from the face of the wafer 12.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 xcexcm. Focusing photo-patterns of such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Thus, CMP processes must create a highly uniform, planar surface.
One problem with CMP processing is that the planarized surface of the wafer may not be sufficiently uniform across the whole surface of the wafer. The uniformity of the planarized surface is a function of the distribution of slurry under the wafer, the relative velocity between the wafer and the polishing pad, the contour and condition of the polishing pad, the topography of the front face of the wafer, and several other CMP operating parameters. In fact, because the uniformity of the planarized surface is affected by so many different operating parameters, it is difficult to determine and correct irregularities in specific operating parameters that adversely affect the uniformity of a given processing run of semiconductor wafers. Therefore, it would be desirable to develop a CMP machine and process that compensates for irregular operating parameters to enhance the uniformity of finished wafers.
In the competitive semiconductor industry, it is also desirable to maximize the throughput of finished wafers. One factor that affects the throughput of CMP processing is the ability to accurately stop planarizing a given wafer at a desired endpoint. To determine whether a wafer is at its desired endpoint, conventional CMP processes typically stop planarizing the wafer and measure the change in thickness of the wafer with an interferometer or other distance measuring device. If the wafer is under-planarized, CMP processing is resumed and the wafer is periodically measured until the wafer reaches its desired endpoint. If the wafer is over-planarized, the wafer may be partially or fully damaged. The throughput of finished wafers is accordingly greatly affected by the ability to accurately and quickly determine the endpoint of a specific wafer. Therefore, it would be desirable to develop a CMP machine and process that determines the endpoint of a wafer without stopping CMP processing.
The present invention is a planarizing machine and method for uniformly planarizing a surface of a semiconductor wafer and accurately stopping CMP processing at a desired endpoint. In one embodiment, a planarizing machine for removing material from a semiconductor wafer has a platen mounted to a support structure, an underpad attached to the platen, a polishing pad attached to the underpad, and a wafer carrier assembly. The wafer carrier assembly has a chuck with a mounting cavity in which a wafer may be mounted, and the wafer carrier assembly moves the chuck to engage a front face of the wafer with the planarizing surface of the polishing pad. The chuck and/or the platen move with respect to each other to impart relative motion between the wafer and the polishing pad. The planarizing machine also has a pressure sensor positioned to measure the pressure at an area of the wafer as the platen and/or the chuck move and while the wafer engages the planarizing surface of the polishing pad. The pressure sensor is preferably one or more piezoelectric sensors positioned in either the underpad, the polishing pad, or the mounting cavity of the chuck. The pressure sensor generates a signal in response to the measured pressure that corresponds to a planarizing parameter of the wafer.
In a preferred embodiment, the planarizing machine further includes a converter operatively connected to the pressure sensor and a controller operatively connected to the converter. The converter transposes an analog signal from the pressure sensor into a digital representation of the measured pressure, and the controller controls an operating parameter of the planarizing machine in response to the digital representation of the measured pressure.
In one particular embodiment of the invention, the planarizing machine further comprises a plurality of actuators operatively connected to the controller and positioned in the mounting cavity of the chuck to act against the backside of the wafer. The pressure sensor is preferably positioned in either the underpad or the polishing pad so that the wafer passes over the pressure sensor. In operation, the pressure sensor generates a signal corresponding to the contour of the front face of the wafer, and the controller selectively drives each actuator toward or away from the backside of the wafer to selectively deform the wafer in response to the measured contour of the front face.
In still another particular embodiment of the invention, the pressure sensor is a piezoelectric stress sensor that is positioned in the mounting cavity of the chuck and releasably adhered to the backside of the wafer. The stress sensor measures torsional stress across an area of the backside of the wafer and generates a signal corresponding to the measured stress. It is expected that changes in stress will indicate an endpoint of the wafer. In operation, the controller stops the planarization process when the measured stress indicates that the wafer is at a desired endpoint.